With the advance of flat panel displays (e.g., liquid crystal displays, plasma displays, and organic EL displays), infrared sensors, optical waveguides, etc., optical material used therein has been demanded to be not only excellent in transparency but also have high heat resistance and high optical isotropy (so-called low birefringence), in recent years.
For example, increase in the sizes of flat panel displays also results in increase in the sizes of necessary moldings for optical material. Due to biased external forces, birefringence distribution takes place, causing the problem of contrast reduction. Thus, material that has small birefringence change caused by external forces, i.e., a small absolute value of a photoelastic coefficient, has been demanded for reducing birefringence distribution.
Moreover, with increase in the sizes of flat panel displays, they have increasing opportunities to be viewed not only from the front but also sideways. In such a case, the displays, in principle, present the problem of display color change or contrast reduction depending on the angle at which they are viewed. Thus, material that has a small absolute value of birefringence has been demanded for improving viewing angles.
Methacrylic resins typified by methyl methacrylate homopolymers (PMMAs) are excellent in transparency and have small birefringence. Therefore, they have been used as material having optical isotropy in various optical applications. However, material properties currently required for the market have shifted to minimized birefringence, smaller birefringence change caused by external forces, and higher heat resistance. The development of material that simultaneously satisfies these properties has been awaited (see Non-Patent Documents 1 and 2).
A ternary copolymer which comprises, of four monomers constituting a thermoplastic acrylic resin of the present invention, methyl methacrylate, styrene, and maleic anhydride except for benzyl methacrylate is disclosed in, for example, Patent Documents 1 to 3, as the related art to the present invention. Patent Document 1 states that a weight ratio (a/b) of a content (a) of the repeating unit derived from a vinyl aromatic monomer to a content (b) of the cyclic acid anhydride repeating unit in the ternary copolymer is preferably not smaller than 1 and smaller than 3, in terms of heat distortion resistance, weather resistance, etc. On the other hand, Patent Document 2 makes mention of neither the content ratio (a/b) nor effects expected from the range thereof and merely states a ternary copolymer having an a/b ratio larger than 1 (a/b=14/10) as Example. Likewise, Patent Document 3 makes mention of neither the weight ratio (a/b) of a content (a) of the repeating unit derived from a vinyl aromatic monomer to a content (b) of the cyclic acid anhydride repeating unit nor effects expected from the range thereof and merely states a ternary copolymer having an a/b ratio larger than 1 (a/b=15/12) as Example.
Moreover, Patent Document 4 discloses a copolymer comprising methyl methacrylate and any one or more of copolymerizable monomers such as styrene, benzyl methacrylate, and maleic anhydride. Patent Document 4 discloses the copolymer as one of compositions constituting an antistatic thermoplastic laminate and does not make any mention of optical properties. Furthermore, the document does not show Example corresponding to the quaternary copolymer of the present invention.
Moreover, Patent Document 5 discloses a copolymer comprising styrenes, maleic anhydrides, and methacrylates. Specifically, Patent Document 5 states that methyl methacrylate and benzyl methacrylate may be copolymerized as the methacrylates in the copolymer. However, the document does not show Example corresponding to the quaternary copolymer of the present invention comprising methyl methacrylate, styrene, benzyl methacrylate, and maleic anhydride. Furthermore, it also states that esters comprising a lower alkyl group are preferable as the methacrylates and does not give any suggestion about the repeating unit derived from an aromatic group-containing methacrylate monomer of the present invention. Furthermore, it also states that a copolymer in which a portion or the whole of the constitutional unit maleic anhydride has been hydrolyzed is a preferable resin.
Moreover, Patent Document 6 discloses a copolymer mainly composed of monomers selected from styrenes, maleic anhydrides, and methacrylates. Examples of the monomers of methacrylates disclosed therein include methyl methacrylate and benzyl methacrylate. Specifically, Patent Document 6 discloses a blend of a copolymer comprising styrenes and methacrylates and a copolymer comprising maleic anhydrides and methacrylates and a blend of a copolymer comprising styrenes and maleic anhydrides and a copolymer comprising maleic anhydrides and methacrylates and does not make any mention of effects obtain by the simultaneous copolymerization of three or more monomers. Particularly, the document does not show Example corresponding to the quaternary copolymer of the present invention. It also states that esters comprising a lower alkyl group are preferable as the methacrylates in the copolymer and does not give any suggestion about the repeating unit derived from an aromatic group-containing methacrylate monomer of the present invention. Furthermore, it also states that a copolymer in which a portion or the whole of the constitutional unit maleic anhydride has been hydrolyzed is a preferable resin.
Furthermore, Patent Document 7 discloses a copolymer of maleic anhydride and acrylate. Specifically, Patent Document 7 states that methyl (meth)acrylate and benzyl (meth)acrylate may be used in combination as the acrylate monomers in the copolymer and that styrenes may be copolymerized therewith as additional monomers without impairing heat resistance. However, the document does not show Example corresponding to the quaternary copolymer of the present invention.